Network Technical Reference - ZyXEL Communications ISG50-ISDN User Manual

27.9 Network Technical Reference

This section contains background material relevant to the Server screens.
ISDN Overview
ISDN (Integrated Service Digital Network) is a circuit-switched telephone network system. In ISDN,
there are two types of channels: B-channels and D-channels. ISDN allows digital transmission of
voice, video and data over ordinary telephone copper wires using B-channels with 64 kbps
bandwidth. D-channels are mainly used for signaling and control with 16 kbps or 64 kbps
bandwidth depending on service levels.
The ISG50 supports the BRI ISDN service level. BRI (Basic Rate Interface) contains two 64 kbps B
channel and one 16 kbps D channel (2B+1D). A BRI user can have up to 128 kbps service.
DiffServ
DiffServ (Differentiated Services) is a class of service (CoS) model that marks packets (based on
the application types and traffic flow) so that they receive specific per-hop treatment at DiffServ-
compliant network devices along their route. Packets are marked with DiffServ Code Points (DSCPs)
indicating the level of service desired. This allows the intermediary DiffServ-compliant network
devices to handle the packets differently depending on the code points without the need to
negotiate paths or remember state information for every flow. In addition, applications do not have
to request a particular service or give advanced notice of where the traffic is going.
DSCP and Per-Hop Behavior
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each packet
gets across the DiffServ network. Based on the marking rule, different kinds of traffic can be
marked for different forwarding treatment. Resources can then be allocated according to the DSCP
values and the configured policies.
The ISG50 allows you to mark outgoing frames with following PHB classifications:
Assured Forwarding (AF) - This is a PHB group allowing you to choose from one of four classes
of forwarding assurance. Each class has three choices of drop precedence to choose from. When
congestion occurs, the DS nodes on your network treat all IP packets marked with the same class
and drop precedence identically. DS nodes can be configured to protect packets with lower drop
precedence value by discarding packets with a higher drop precedence value.
Expedited Forwarding (EF) - The EF PHB defines a single DSCP designed for simulating a private
end-to-end pipeline across a DiffServ network. IP packets are marked for high priority low-loss,
low-latency, low-jitter, assured-bandwidth end to end service through DS domains. This is
recommended for use with VoIP traffic as it addresses the factors that cause degradation in sound
quality.
Class Selector (CS) - IP packets are marked with values partially backwards-compatible with
known current uses of IP precedence field. The packets marked with higher values are given higher
priority than those with lower values.
ISG50 User's Guide
Chapter 27 Global PBX Settings
447